1. Field of Invention
The present invention relates to an alignment mark and a method for forming the same. More particularly, the present invention relates to an alignment mark with a relatively high contrast under an optical tool and a method for forming the same.
2. Description of Related Art
In the manufacture of integrated circuit, photolithography process is used to transfer patterns from a photo mask having customized circuit patterns to thin films formed on a wafer. The image transfer process comprises steps of forming a photoresist layer on a non-process layer, illuminating the photoresist layer through a photo mask having the customized circuit patterns, developing the photoresist layer and then etching the non-process layer by using the patterned photoresist layer as a mask. Hence, the image transfer process is accomplished. For a well-manufactured integrated circuit product, the image transfer process mentioned above is performed several times to transfer the circuit patterns to each non-process layers to form the electrically circuit device.
During the photolithography process for transferring the pattern from photo mask to the target material layer, it is necessary for the exposure tool to position the wafer on the station by referring the alignment mark on the wafer. FIG. 1 is a diagram schematically illustrating a conventional alignment mark. FIG. 1A is a top view showing a wafer with a plurality of device regions thereon. FIG. 1B is an enlarge view of a portion of FIG. 1A for illustrating one of the alignment regions of the wafer shown in FIG. 1A. As shown in FIG. 1A together with FIG. 1B, conventionally, in order to achieve proper alignment, at least one alignment mark 106 is formed at the alignment region 104 aside the device region 102 on the wafer 100. FIG. 1C is an enlarge view of the alignment region shown in FIG. 1B. FIG. 1D is a spectrum showing an image contrast of the conventional alignment mark shown in FIG. 1C. As shown in FIG. 1C, for each alignment mark 106, there are several sub-marks 106a. Each sub-mark 106a composed of a fine frame 108 and an alignment component 110 which is enclosed by the fine frame 108. Conventionally, the size of the fine frame 108 is smaller than or equal to the size of the spacer (not shown) formed on the device element (not shown) in the device region 102 at the same material level of the fine frame 108. Hence, the surface area of fine frame 108 is only about 0.1% of the sub-mark 106a for single sub-mark 106a. Therefore, as shown in FIG. 1D together with FIG. 1C, the fine frame 108 contributes to a very low the image contrast according to the background material layer 112. That is, it is hardly recognize the fine frame 108 for the alignment tool to position the wafer. Therefore, the alignment preciseness for later performed photolithography process is poor.